ginzton



E. L. GlNzToN LINEAR ACC'ELERATOR Aug. 11, 1959 2 Sheets-Sheet 1 vx-ild may 28, 1956 Aug. 11, 1959 E. L. GlNz-roN 2,899,598

l LINEAR ACCELERATOR Filed May 28, 1956 2 Sheets-Sheet 2 LINEAR ACCELERATOR Edward L. Ginzton, Los Altos, Calif., assignor to The Board `of Trustees of The Leland Stanford Jr. University, Stanford, Calif., trustees Application May 28, 1956, Serial No. 587,543

15 Claims. (Cl. S15-3.6)

This invention relates to linear accelerators for accelerating charged sub-atornic particles, usually electrons, and in particular to improvements in such accelerators for inhibiting the undesired production of X-rays, neutrons, and the like, and for other purposes.

A linear accelerator is a device for accelerating charged sub-atomic particles, usually electrons, by means ofthe interaction between a beam of such particles and electromagnetic waves transmitted by a suitable transmission structure lwith a phase velocity substantially equal to the velocity of the particles that are accelerated. Since the required phase velocity is somewhat smaller than the free-space velocity of electromagnetic waves, the transmission structure is sometimes called a slow-Wave structure. Generally it is an evacuated tubular waveguide containing a plurality of longitudinally spaced loading discs.

A beam of charged particles, usually electrons, provided by any suitable means, is directed longitudinally through the waveguide along its central axis. Electromagnetic energy is supplied to the waveguide for providing therein electromagnetic waves having a phase velocity substantially equalfto the velocity of the particles. The loading discs have axially alined central apertures for transmitting the beam of particles and the electromagnetic waves. By proper choice of Vwaveguide and aperture dimensions, the spacing of the loading discs, the frequency of the electromagnetic energy, and the initial velocity of the particles, interaction between the'electromagnetic waves and the beam may accelerate some of the charged particles to high energy values. The design and construction of linear accelerators is already yknown to those skilled in the art, and a detailed description thereof in this patent application would be superuous.

For efficient transmission of electromagnetic energy without excessive losses, the waveguide and the loading discs are made of a material having good electrical conductivity, usually copper or silver. Copper and silver are metals having fairly large atomic numbers, 29 for copper and 47 for silver, and these metals yield an abundance of X-rays (or 7 rays) when bombarded by highvelocity electrons.

In conventional linear accelerators, the waveguide loading discs are bombarded by many high-velocity electrons, because some Idefocusing or spreading of the electron beam is inevitable, and also because ystray electrons (for example, electrons emitted from the Iwalls of the waveguide and loading discs) may be accelerated along the lengthwise direction of the waveguide. Consequently the accelerated electrons provided at the output end of the accelerator are accompanied by a substantial amount of X-radiation, which often is very undesirable.

Furthermore, bombardment of the waveguide walls and yloading discs by high-energy electrons may produce radioactive products that continue to emit dangerous radiations for a considerable time after the electron beam i'sshut olf, in consequence of'which it may be unsafe for the operator to approach the accelerator for some time vnited States Patent' after a run or operation ofthe accelerator' is'V com` pleted. y

Furthermore, when the accelerator accelerates electrons to energies exceeding about l5 m.e.v. (million electron volts), and such electrons bombard parts of the waveguide or loading discs, there are produced X-rays (or y rays) of such energy that the X-rays (or y rays) may interact with copper atoms to produce neutrons that also accompany the accelerated electrons provided at the output of the accelerator.

Accordingly, a principal object of this invention is to provide means in alinear accelerator for substantially reducing bombardment of the waveguide walls and loading discs by high-velocity electrons and other particles, to inhibit the undesired production of X-rays,rneutrons, and the like. Other objects are to provide a better delined and more homogeneous beam of particles at the output end of the accelerator. Still other objects and advantagesl will appear as the description proceeds.

Briefly stated, in accordance with certain aspects off this invention, one or more partitions are provided within the waveguide of a linear accelerator. These partitions divide thewaveguide into aplurality of longitudinally alined waveguide sections. Each partition-has a central aperture alined withy the velectron beam of the accelerator so that a central portion of the beam passes through the apertures. Other portions of the partitions are substantially impermeable to the electrons, and restrict the crosssectional area of the electron beam. The apertures of the partitions are substantially smaller than the apertures of the waveguide loading discs, so that few if any electrons of the restricted 'beam bombard the loading discs of the waveguide. To prevent neutron formationby stray electrons that may bombardthe waveguide walls and the loading discs, the length of each waveguide section between the partitions is made less than the distance within which electrons.V or other charged particles are accelerated l5 m.e.v. by the accelerator. If desired, the partitions may be even moreclosely spaced, Y

Althoughthepartitions prevent substantial bombardment of the waveguide walls and loading disc s,vthe` partitions themselves larefsubject to considerable electron bombardment. To inhibit the productionF'of X-rays -by such bombardment,` the partitions are made` of a material that does not yield an abundance of X-rays -when bombarded by electrons In general, the yield of X-rays under electron bombardment of an element is a direct functionl of the atomic number of that element. Therefore, :the partitions should ,not contain anyv element having a large atomic number, and preferably they are made of a ,metal having an atomic number smaller than about 14. On the other hand, the depth of material that electrons of a given velocity can penetrate is an inverse function of its atomic number. Since portions of the partitions other than their central apertures should be substantially impermeable to the electrons present atV that location in' the accelerator, it is desirable that the partitions should be of a material having as high an atomic number as is permissible in view of other considerations involved, so that the thickness of the partitions will notbe excessive.

Fortunately, aluminum, having an atomic number of 13, satisfactorily meets all of the requirements. Its yield of X-rays under electron bombardment is low, its electrical conductivity is suiliciently good that an aluminum partition doesnot produce excessive losses of electromagnetic energy in the waveguide, and its penetrability by-ele'ctrons is reasonably small. Furthermore, aluminum is readily available, durable, and easy to fabricate intofrequired shapes. Another advantage of aluminum is that the radioactive product nucleus produced by electron bombardment of aluminum has a half-life of approximately 7 seconds, while that produced by electron bombardment of copper has a half-life of approximately 12.8 hours. Consequently, any dangerous radiations emitted byA the bombarded aluminum partitions quickly die out after the electron beam is cut off, so that the operator can safely `approach the accelerator soon after a run is completed.

The partitions divide the accelerator waveguide into a plurality of longitudinally alined waveguide sections. Electromagnetic waves should be provided in each waveguide section, and the waves in each waveguide section must be properly related in phase to maintain an electron-accelerating phase relation between the electromagnetic waves in each waveguide section and the accelerated electrons. Preferably electromagnetic energy is supplied to one of the waveguide sections for producing electromagnetic waves therein, and conventional coupling circuits are provided for transmitting electromagnetic waves from one side to the other of` each partivtion in proper phase to maintain the correct phase relabetter defined. Furthermore, the beam is more hoinoo geneous, that is, all of the electrons are more nearly equal in energy, since the partitions remove from the beam stray electrons that have not been fully accelerated. Removal of the stray electrons may also increase the efficiency of the accelerator, since electromagnetic energy is not used in further acceleration of undesired particles.

The invention will be better understood from the following detailed description taken inrconnection with the accompanying drawings, and Yits scope will be pointed out in the appended claims. In the drawings:

Fig. 1 is a partly schematic longitudinal section of a linear accelerator embodying principles of this invention;

Fig. 2 is a transverse section taken along the line 2-2 of Fig. l;

Fig. 3 is a fragmentary longitudinal alternative embodiment;

Fig. 4 is a fragmentary longitudinal section of another alternative embodiment;

5 is a partly schematic fragmentary longitudinal section of another alternative embodiment;

Fig. 6 is a partly schematic longitudinal section of still another alternative embodiment; and

Fig. 7 is a diagram illustrating relative depths of penetration of electrons and X-rays.

Referring to Figs. l and 2 of the drawings, a linear accelerator includes an evacuated tubular waveguide 1 containing a plurality of longitudinally spaced transversely disposed loading discs, identified in the drawing by reference numerals 2 through 9, forming a slow-wave structure for the transmission of electromagnetic waves with phase velocities somewhat smaller than the freespace velocity of electromagnetic waves. The waveguide and loading discs are made of a materialhaving good electrical conductivity, usually copper or silver. An `electromagnetic wave source 10, whichfmay be any suitable generator of microwave frequency, electromagnetic oscillations, supplies electromagnetic energy through transmission line 11 and coupling loop 12 to the waveguide for providing electromagnetic waves therein. An electron beam `source 13, which may be any suitable apparatus for producing abe-am of electrons traveling at the desired initialvelocity, and which may include other accelerator sections, supplies a 'beam of electrons, represented in the drawing by broken lines 14, travelsection of an 4 ing longitudinally through the waveguide substantially along its central longitudinal axis.

The loading discs 2 through 9 have axially alined central apertures, as shown, for transmitting the electron beam and electromagnetic waves. By proper choice of the waveguide and aperture dimensions, the spacing of the loading discs, the initial velocity of the electrons, and the frequency of the electromagnetic energy,

` in accordance with principles well known to those skilled in the art, the phase velocity of the electromagnetic waves transmitted by the waveguide can be made substantially equal to the velocity of the electrons, and interaction will occur between the electromagnetic waves and the beam to accelerate some of the electrons to high energy values. It will be understood that the waveguide may be of any desired length, and that a relatively short waveguide, containing a small number of loading discs, is shown in the drawing by way of illustration only.

A diaphragm 1S at the right or output end of wave-v` guide 1 contains ari electron permeable window, usually a thin sheet of aluminum or the like, through which the accelerated electrons may pass for utilization in any desired manner. If desired, diaphragm 15 may be omitted, and an evacuated chamber may be provided at the output end of the accelerator for utilization of the electronk beam. Since the design and construction of linear accelerators is already known tofthose skilled in the art, these conventional accelerator parts will not be described in detail.

The present invention relates to the beam-restricting means that will now' be described. Within waveguide 1, and between its ends, there is a transverse partition 16, preferably made of aluminum, having a central aperture 17. Aperture 17 is alined with the axis of the electron beam for transmitting or conveying a central portion thereof, and is substantially smaller than the apertures of loading discs 2 through 9. The 'thickness of partition 16 is substantially equal to or greater than the penetration depth of the partition material by electrons having the maximum electron velocity at that location in the accelerator. Consequently, those portions of partition 16 other than the apertures therein are substantially impermeable to electrons, and the partition acts to restrict the cross-sectional area ofthe electron beam to the area of aperture 17.

Other partitions similar to partition 16 may if desired be provided at other locations along the length of the waveguide for repeated restriction of the cross-sectional area of the beam. A plug 18, also preferably made of aluminum and containing a central aperture 19, may be provided at the output end of waveguide 1 for restricting the cross-sectional area of the electron beam just before it is utilized.

Restriction of the electron beam by partition 16 to a cross-sectional area substantiallysmaller than the central apertures in the loading discs prevents the bombardment of loading discs 6, 7, 8 and 9 by high-energy electrons of the beam despite any slight spreading or defocusing of the beam that may occur between partition 16 and the output end of Iwaveguide 1. Substantial bombardment of loading discs 2,3, 4, and 5 is prevented by initial concentration of the electron beam before it leaves electron beam source 13, which may, if desired, also be accomplished by an apertured partition or mask.

In case of very long accelerator sections, one intermediate partition, such as partition 16, may not be adequate to prevent excessive spreading of the beam. In this case, additional partitions may be added at spaced intervals along the length of the accelerator. In this way substantial bombardment of the loading discs by high-energy electrons of the beam is prevented, and the production of X-rays, neutrons and other undesired radiations ,and particles is inhibited.

Althopgh the apertured partitions prevent substantial bombardment of the loading discs and waveguide lwalls by high-energy electrons, the partitions themselves are subject to substantial electron bombardment. It is Iwell known thatthe yield of X-rays'obtained upon bombardment of a target by electrons is directly related to the atomic number of the target, andthat the yield'of X-rays increases as the atomic number is made larger. Consequently, the amount of X-radiation produced bythe electron bombardment of partition 16 and plug -18 can be kept small by making these parts of elements having low atomic numbers, preferably smaller than 14.

On the other hand, the vdepth of penetration by the electrons bombarding any member is inversely related to the atomic number of that member. Since partition 16 and plug 18 should be substantially impermeable to electrons, and yet should not be excessively thick, it is desirable that these parts be made of a material having as high an atomic number as is consistent with other considerations. Furthermore, these parts should have a fairly low electrical resistivity to prevent excessive losses of electromagnetic energy in the waveguide, they should be of materials that are readily available and easily formed into the required shapes, and they should be stable under conditions encountered in manufacture and operation of the accelerator, particularly in a high vacuum and at a moderately high temperature required for outgassing during evacuation. Any radioactive products formed by electron bombardment of the partitions should have short half-lives so that any dangerous radiations emitted thereby will quickly die out after the electron beam is cut off. Fortunately, aluminum satisfactorily meets all of the requirements, and therefore partition 16 and plug 18 preferably are made of aluminum, although other materials, such as magnesium, might prove satisfactory in some instances.l

Even though no electrons of beam 14 strike the loading discs, the walls and the loading discs of the waveguide may be bombarded by stray electrons emitted from other parts of the waveguide, such as from other loading discs. Some of these strayelectrons may be accelerated by the electromagnetic waves in the waveguide, and may travel down the waveguide and acquire high velocities. The production of high energy X-rays by such stray electrons is prevented'v by spacing partitions 16 ysuliciently close together that yall electrons outside the centralpor- -tion of the electron beam 14vare intercepted by one of the partitions before they can be accelerated to undesirably high velocities.

Bombardment of the loading discs by electrons having energies in excess of l5 m.e.v. is especially undesirable, since such bombardment may produce very energetic X- rays (radiations of such high energy are sometimes arbitrarily called y rays, but except for wavelength, they are like X-rays and are herein called X-rays) capable of neutron production. Accordingly, the vlength of each waveguide section between lsuccessive beam-restricting partitions is made smaller than the accelerator length within which particles4 may be accelerated by l5 m.e.v.

For accelerating the electrons or other charged particles of beam 14, electromagnetic waves of proper phase should be provided in each .section of the waveguide.A Referring to Fig. l, electromagnetic energy is supplied tothe leftl or entrance end of the waveguide by electromagnetic wave source 10, and electromagnetic waves are transmitted from left to right in the waveguide with a phase` velocity substantiallyV equal 'tothe'velocityof .the particles in beam 14. The central apertures vot'v loading discs` vn2 through 9 provide capacitive coupling for transmitting electromagnetic waves froml one" side to the other of each loading disc. However,.because of its frelatively long length and small diameter, aperture 17 d oes not provide sufficient capacitive coupling for `an` adequate transmission of electromagnetic waves from one side to ythe other of partition v16, and other means must be provided for sup- 6 plying sufficient electromagnetic energy to maintain eleci4 tromagnetic waves in the right or output end of waveguide 1.

In the apparatus shown in Figs. 1 and 2, this is accomplished by providing other apertures 20 and 21 through non-central portions of partition 16, as shown in the drawings. As will readily be understood by those skilled in the art, apertures 20 and 21 provide inductive coupling for transmitting electromagnetic energy from one side to the other of partitition 16. Apertures 20 and 21 are out of the electron beam path, and are for the sole purpose of transmitting electromagnetic energy. In addition to inductive apertures, other coupling circuits, examples of which are hereinafter described, may also be employed for similar purposes.

It is necessary that the phase relations of the electromagnetic waves in the various waveguide sections be such that the electromagnetic waves throughout the length of the waveguide are in substantially electron-accelerating phase relation to the same electrons of beam 14. Assume, for example, that the spacing of the loading discs in each waveguide section is so chosen that the accelerated electrons in beam 14 travel through a distance equal to four times the spacing between loading discs during one cycle of the electromagnetic energy supplied by source 10. Then, to maintain an electron-accelerating relation throughout the length of the waveguide section between the electromagnetic Waves in the waveguide and the accelerated electrons, the phase angles of the electromagnetic waves must advance by degrees in each successive length of the waveguide section equal to the spacing between adjacent loading discs. As is well known to those skilled in the art, such a phase relation can be achieved by proper choice of the aperture size of the loading discs and the frequency of the electromagnetic energy.

Now assume that the thickness of partition 16 is substantially equal to twice the loading disc spacing, and that the loading discs 5 and 6 upon either side of partition 16 are spaced therefrom by a distance substantially equal to the loading disc spacing. To maintain the proper phase relation between electromagnetic waves and the accelerated electrons, the phase of the electromagnetic waves must be advanced' by 270 degrees, or retarded by 90 degrees, in passing through partition 16. This is achieved by a proper choice of the size and shape of apertures 20 and 21 to provide the correct amount of inductive coupling for producing a phase lag of 90 degrees. In practice, the size and shape of apertures 20 and 21 is usually determined experimentally, by constructing partitions with apertures of different sizes and measuring the relative phase angles of electromagnetic waves on each side of the partition. v

The thickness of partition 16 is not necessarily equal to twice the spacing of the loading discs, but may be more or less than this amount depending upon the maximum velocity of electrons encountered at that location in the accelerator. Generally partition 16 is made just sucient for the partition to be impermeable to any electrons except lthose passing through aperture 17, although of course the thickness of partition 16 may be greater if for any reason such is desired. The thickness of the partition and the velocity of the accelerated electrons then determines the phase relation that must exist between electromagnetic waves on each side of the partition, and a coupling circuit is then designed to give this phase relation. Such design will present no particular problems `to those skilled in the art, since almost any desired phase relation can be obtained by appropriate amounts of inductive or capacitive coupling, and the design of coupling circuits is well known.

Instead of or in addition to inductive coupling apertures` for transmitting electromagnetic waves from one side to the other of the beam-restricting partition, coupling circuits of other types known to those skilled in the art may be employed. Fig. 3 illustrates `one such alternative coupling circuit. In Fig. 3, which is a fragmentary longitudinal section of a linear accelerator waveguide, a hollow tubular waveguide 22 contains a plurality of conventional loading discs, such as discs 23 and 24. A beam-restricting aluminum partition 25 has a central aperture 26 alined with and smaller than the central apertures of the loading discs for restricting the cross-sectional area of the` electron beamof the accelerator. Electromagnetic energy is transmitted from one side to the other of partition 25 by means of a coupling circuit comprising a wire 27 or other electrical conductor extending through an off-center hole in partition 2S, as shown, to form a coaxial transmission line. At opposite ends of wire 27 there are two electromagnetic coupling loops 28 and 29, having similar electromagnetic polarities. This circuit provides inductive coupling between electromagnetic waves on opposite sides of partition 25, and transmits electromagnetic energy from one side to the other of the partition.

Another alternative coupling circuit is illustrated in Fig. 4. In Fig. 4, a waveguide 30 for a linear accelerator contains a plurality of conventional loading discs such as discs 3l and 32. An aluminum partition 33 has a small central aperture 34 for restricting the cross-sectional area of the electron beam. Being designed for an accelerator location where the maximum electron velocity is relatively low, partition 33 is much thinner than the partition shown in Figs. l and 3, a-nd a capacitive coupling is needed to give the proper phase relation between electromagnetic waves on opposite sides of the partition.` This capacitive coupling is provided by a coupling circuit comprising a wire 35 extending through an ott-center hole in the partition, as shown. Coupling loops 36 and 37 at opposite ends of the wire 35 have opposite electromagnetic polarity. Consequently, although the coupling loops couple the magnetic ields of the electromagnetic waves, the opposite polarities of the two loops produces a phase reversal so that, insofar as phase relations are concerned, the coupling circuit provides essentially capacitive coupling between the electromagnetic waves.

If desired, an external coupling circuit may be employed for transmitting electromagnetic energy from one side to the other of the partition, as is shown in Fig. 5. Referring now to Fig. 5, a waveguide 38 for a linear accelerator contains a plurality of conventional loading discs, such as discs 39 and 40. An aluminum partition 41 has a relatively small central aperture 42 for restricting the cross-sectional area of the electron beam. Electromagnetic energy is transmitted from one side to the other of partition 41 by means of coupling loops 43 and 44 connected together by coaxial transmission lines 45 and 46 and a phase adjustment circuit 47. The phase adjustment 47 may be designed or adjusted to give any desired phase relation between the electromagnetic waves on opposite sides of partition 41.

Instead of supplying electromagnetic energy to only one waveguide section, and transmitting electromagnetic waves from this section to other sections of the waveguide, as hereinbefore explained, electromagnetic energy of proper phase may be supplied separately to each section of the waveguide, as is illustrated in Fig. 6.

Referring now to Fig. 6 of the drawings, a linear accelerator comprises an evacuated hollow tubular waveguide 48 containing a plurality of loading discs identified in the drawing by reference numerals 49 through 57. Any suitable electron beam source 58 provides a beam of electrons traveling lengthwise through the waveguide substantially along its central longitudinal axis and emerging from the outlet end of the waveguide through an electron permeable window in a diaphragm 59. The cross-sectional area of the electron beam is successively restricted by an aluminum partition 60 having a relatively small central aperture 61, another aluminum partition 8, 62 having a small centralaperture 63, and an aluminum plug 64 at the output end of the waveguide having a small central, aperture `6 5.

The partitions 6.0 a11d;62` and the plug 64 have difterent thicknesses corresponding to differences in the maximum electron velocities encountered at their respective locations in the waveguide, so that the thickness of each partition is just sufcient to make it substantially impermeable to the electrons that impinge thereon. Partitions 60 and 62 divide waveguide 48 into three longitudinally alined waveguide sections. Electromagnetic energy is supplied separately to each of the three waveguide sections for providing therein electromagnetic waves in proper phase relation to accelerate the same electrons of the electron beam.

Electromagnetic energy is supplied by any suitable electromagnetic wave source 66. A portion of this electromagnetic energy is transmitted through coaxial transmission line 77 and coupling loop 78 to the waveguide section at the left of input end of the accelerator. Another portion of the electromagnetic energy supplied by source 66 is transmitted through coaxial transmission line 79, phase adjustment 80 and coupling loop 81 to the center waveguide section. Still another portion of the electromagnetic energy is transmitted through transmission line 82, phase adjustment 83 and coupling loop 84 to the waveguide section at the right or output end of the accelerator.

In this way electromagnetic waves are provided in all three of the waveguide sections. By proper design or adjustment of the phase adjustments 80 and 83, the phase relations of the electromagnetic waves in the three waveguide sections can be so related to the electron velocities that the same electrons of the beam are accelerated by electromagnetic waves in all three of the waveguide sections, and very high-energy electrons are supplied at the output end of the accelerator. j

The present invention is useful in any case `where it is desirable to provide a homogeneous and well-defined beam of high energy electrons, relatively free from X-rays, neutrons, and other undesired radiations or particles. It may be especially useful, for example, in medical applications where electrons are used to destroy abnormal tissue with minimum damage to healthy tissue. In such cases, the presence of undesired X-rays is especially objectionable.

Referring to Fig. 7, solid curve 85 represents intensity versus depth of penetration of an electron beam used to destroy abnormal tissue. Assume, for example, that a cancer or other abnormal growth that is to be destroyed is located in the body at a depth represented in Fig. 7 by point 86. Further assume that some vital organ, which must not be damaged, is located at a depth represented by point 87. By proper adjustment of the electron energies, the electron beam can be made to penetrate just beyond point 86, to produce maximum effect in destroying the cancer, without producing any substantial effect upon the organ located at point 87.

On the other hand, the intensity of X-rays as a func tion of the depth of penetration is a logarithmic function, as is represented in Fig. 7 by broken line 88. If large amounts of X-radiation accompanies the electron beam, it is evident that a substantial portion of this X-radiation will penetrate to the vital .organ located at the depth represented by point 87, and that this organ may be seriously damaged by the X-rays.

The present invention, which inhibits the production of X-rays in the linear accelerator and provides an electron beam that is substantially free of X-radiation, thus provides a means for the treatment or destruction of abnormal tissue under circumstances and with a degree of safety heretofore unobtainable. However it will be realized that this invention is not limiteclto medical applicatons, but that it is also useful in physical and other scientific investigations wherein relativelypure beams of 9. electrons or other charged particles are desired, and inother applications wherein any advantage provided by this invention, ysuch as the rapid decay of dangerous radiations after the electron beam is cut of, is desired.

It should be understood that this invention in its broader aspects is not limited to specific embodiments herein illustrated and described, and Vthat thefollowing claims are intended'to cover allchanges andfmodications that do not depart from theptrue spirit and scope of the invention.

What is claimed is: l v

1. A hollow Waveguideffor-lnear accelerators, said waveguidecontaining a plurality of longitudinally spaced loading discs and a transversel partition,favplurality of said discs being disposed, on each side of-said partition, said discs and Sai-d partition having alined apertures for transmitting a beam of particles traveling lengthwise through said waveguide and for transmitting electromagnetic wave-energy through said waveguide on both sides of said partition in interacting relation with said beam, said aperture of the partition being smaller than said apertures of the loading discs for restricting the crosssectional area of said beam. 1

2. A linear accelerator Vfor accelerating charged. subatomic particles, comprising a hollow waveguide, means for providing a beam of said particles traveling length-v wise through'said waveguide, a transverse partition dividing said waveguide into a plurality of longitudinally* alined waveguide sections, each of saidfwaveguide'sections containing a plurality of longitudinally spaced loading discs, said partition and said ydiscs having alined central apertures for transmitting said beam, said aperture of the partition being smaller than said apertures of the loading discs for restricting the cross-sectionalarea of said beam to prevent substantialbombardmentv of said discs by said particles, and meansfor providing electromagnetic waves to accelerate said particles in each of said waveguide sections.

3. A linear accelerator for accelerating charged subatomic particles,comprising'a hollow waveguide, means for providing a beam of'said' particles'traveling lengthwise through said waveguide, Y'a ltransverse partition'jdividing-Said waveguide into V'aplurality of longitudinally alined waveguide sections, each of said waveguide sections containing a plurality of longitudinally spaced loading discs, said partition and said discs having alined central apertures for transmitting said beam, said aperture of the partition being smaller than said apertures of the loading `discs for restricting the cross-sectional area of said beam to prevent substantial bombardment of said discs by said particles, and means for providing electromagnetic waves to accelerate `said particles in each of said waveguide sections, the length of each waveguide section being less than the distance within which said particles are accelerated mev.

4. A linear accelerator comprising means for supplying a beam of electrons, a slow-wave structure for transmitting electromagnetic waves in electron-accelerating relation with said beam, a partition having an aperture axially alined with said beam for conveying a central portion thereof, other portions of said partition being substantially impermeable by said electrons for restricting the cross-sectional area of said beam, said partition being disposed transverse to -and between the ends of said slowwave structure, and means for providing electromagnetic waves in said structure on both sides of said partition.

5. A linear accelerator comprising means for supplying a beam of electrons, a slow-wave structure for transm-itting electromagnetic waves in electron-accelerating relation with said beam, a partition having an aperture axially alined with said beam for conveying a central portion thereof, other portions of said partition being substantially impermeable by said electrons for restricting the cross-sectional area of said beam, said partition being disposed transverse to and between the ends of said slowwave structure, means for supplying electromagnetic energy to said structure, 'and means fortransmitting electromagnetic waves from one Aside to the other of said partition. l Y

6. A linear accelerator comprising means for supplying a beam of electrons, a slow-wave structure for transmitting electromagnetic waves in electron-accelerating relation with said beam, a partition having an aperture axially alined with said beam for conveying a central portion thereof, other portions of said partition being substantially impermeable by said electrons -for restricting the crosssectional area of said beam, said partition being disposed transverse to and between the ends of said kslow-wave structure, and coupling means for transmitting electromagnetic waves through said partition.

7. A linear accelerator comprising means for supplying a beam of electrons, a slow-wave vstructure for transmitting electromagnetic waves in electron-accelerating relation with said beam, a partition having a first aperture axially alined with Vsaid beam for conveying a central portion thereof, other portions of said partition being substantially-impermeable by said electrons for restricting the cross-sectional area of of said beam, said partition being disposed transverse to and between the ends of said slowwave structure, said partition having a second aperture out-of alinement with said beam for transmitting electromagnetic ywai/ esthrough said partition.

8. A linear'accelerator comprising means for supplying a beam of electrons, a slow-wavestructure for transmitting electromagnetic-waves in 'electron-accelerating relation with said beam, a partition having a rst aperture axially alined with said -beam for conveying ay central portion thereof, said Vpartition having a second aperture out of alinement with said beam, a conductor extending through said second aperture, said conductor having loops at opposite ends thereof forming a coupling circuitv for transmitting electromagnetic waves from one side to the other of said partition, other 'portions of said partition being substantially impermeable by said electrons for restricting'the cross-sectional area of said beam.

9. A linear accelerator as in claim 8, in which said conductor has inductive coupling loops of similar electromagnetic polarity at opposite ends thereof.

10. A linear accelerator as in claim 8, in which said conductor has inductive coupling loops of opposite electromagnetic polarity at opposite ends thereof.

ll. A linear accelerator comprising means for supplying a beam of electrons, a slow-wave structure for transmitting electromagnetic waves in electron-accelerating relation with said beam, a partition having an aperture axially alined with said beam 4for conveying a central portion thereof, other portions of said partition being substantially impermeable by said electrons for restricting the cross-sectional area of said beam, said partition being disposed transverse to and between the ends of said slow-wave structure and dividing the same into two longitudinally alined portions, and an external coupling circuit for transmitting electromagnetic waves from a portion of said slow-wave structure on one side of said partition to a portion of said slow-wave ystructure on the other side of said partition.

12. A linear accelerator comprising means for supplying a beam of electrons, a slow-wave structure for transmitting electromagnetic waves in electron-accelerating relation with said beam, a partition having an aperture axially alined with said beam, for conveying a central portion thereof, other portions of said partition being substantially impermeable by said electrons for restricting the cross-sectional area of said beam, said partition bein-g disposed -transverse to and between the ends of said slowwave structure and dividing the same into longitudinally alined wave-transmitting portions, and external circuit means for supplying electromagnetic energy to portions of said slow-wave `structure on both sides of said partition.

13. A linear accelerator for accelerating electrons, comprising an evacuated `hollow y'tubular waveguide, a transverse aluminum partition `dividing said waveguide into two longitudinally alinedwaveguide sections, each of said waveguide sections containing a plurality of longitudinally spaced loading discs, said `discs having axially alned central apertures, said partition having a central aperture axially alined with and smaller than said apertures `of the loading discs, means for providing a beam of electron-s traveling longitudinally through said waveguide and `passing through said apertures, said apertured partition` restricting the cross-sectional area of said beam to prevent substantial electron bombardment of said loading discs, means for supplying electromagnetic energy toa first one of said waveguide sections, and a coupling circuit for transmitting electromagnetic energy from said rst one to the other of said waveguide sections.

14. A linear accelerator for accelerating charged par* ticles, comprising means forsupplying a beam of charged particles that are to be accelerated, meansfory supplying electromagnetic waves, a slow-wave structure for transmitting said waves in particle-accelerating relation to said beam of charged particles, said slow-wave structure containing a material having an atomic number larger than 14, said structure having a lengthwise passage through which said beam passes, said structure having two ends 'through which said beam enters and leaves said passage respectively, said beam spreading transversely as it passes through said passage, and an apertured partition disposed across said passage between said two ends of the slowwave structure, said partition being of a material having an atomic number smaller than 14, said partition having an aperture therethrough in substantial axial alinement with said beam, said aperture having transverse dimensions smaller than the transverse dimensions of said passage, so that a central portion of said beam passes through said aperture while said partition intercepts peripheral portions of said spreading beam and prevents substantial bombardment of said slow-wave structure by 'theaccelerated charged particles, whereby said partition prevents said accelerated charged particles yfrom substantially bombarding said material having an atomic number larger than 14.

15. A linear accelerator for accelerating charged particles, comprising means for supplying a beam of charged particles that are torbe accelerated, means for supplying electromagnetic waves,` aslowwave structurejfor transmitting said waves in particle-accelerating,relation to said beam of charged particles, said structure havinga lengthwise passage through nwhich said beam passes, such-particles being continually accelerated along the length of said structure as they pass through said passage, said structure emitting in s aid passage stray electrons that are also accelerated continually along the length of said structure, the total length of said structure being sufficient to accelerate said stray electrons to velocities greater than 15 `mev., 'said slow-wave structure containing a material that emits'neutronswhen it is bombarded by electrons having velocities substantially greater than 15 mev., and partitioning means dividing said structure into a plurality of lengthwise sections each of insucientlength to accelerate -said stray electrons to velocities `as great as 15 mev., each such `partitioning,means extending across said ypassage `and having an aperture in substantial axial alinement` with said beam to permit a central portionof said beam to pass through said partitioning means, said aperture having transverse dimensions smaller than the transverse dimensions of said passage so that a majority of said stray electrons are intercepted before they are accelerated `to velocitiesas great as l5 mev., said partitioning means being -substantially impermeable to said stray electrons, whereby the emission of neutrons is substantially inhibited.

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